When to Semaphore vs Dispatch Group! Careful Multithreaded Shared Resource

Shared resources are often used across many different tasks in a multithreaded iOS application. If you want to make sure that a share resource is not accessed at the same time, you can guarantee the behavior through a Semaphore. However if your tasks are independent of one another, then using a Dispatch Group is probably the way to go.
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[@TheJhuerta88]

Great video. I like that you're posting more advanced concepts in your channel.

[@romanromanenko9520]

Hey Brain, thanks for the video! You do good job and help people to learn iOS! I have one comment at 9:23 - it's not confusing, it's because you've set DispatchSemafore value to 0. The Semafore value represents how many treads can be executed at one point of time. When you call semaphore.wait(), you decreased value to -1. If semaphore's value is negative integer it is block execution of task, and system will wait when you release tread/treads which are in an awaiting state. What you did by replacing the lines of code, is at first you increased semaphore value to 1 (which is allow to run tasks on 1 tread at one point of time ), then fetchImage() was executing, then you decreased semaphore value back to 0 by calling semaphore.wait(). So, if you set DispatchSemafore value to 1 and return the order of lines of code, when you call at first semaphore.wait() and after fetchImage() semaphore.signal() - it will work.

[@luizfad]

Hi Brian, I can't count the number of times I clarified some concept by watching your videos. Thank you so much for all the work you're doing!

[@mike_thecatlover]

Thanks Brian. Please continue videos like this for advanced stuff. Now too many resources only for beginners.

[@masonictemple3193]

Thank you so much Brian! This video was incredible, I was able to use it to improve the user experience in our app when loading multiple images into a slideshow!

[@neklasoctan8681]

Very detail and clear, Im 3 years iOS developer btw. Thank for give me more detail about this.

[@onurisk5617]

Advanced topics, interesting problems and great solutions. Thank you Brain

[@srikanthburra2512]

Helpful :) Expecting to share a single video which covers all advanced concepts

[@MrBlaq]

Props. I appreciate the advanced tutorials.

[@max500k]

Thanks Brian! Awesome video

[@ΙωάνναΑναστασάκη-ρ7δ]

Amazing video, thank you so much!!!

[@philipplamenov8649]

You are awesome. Many thanks from Bulgaria :)

[@ragnacode56]

Thank you, Brian! Nice video and good explanation of what is going on. Can you please make more videos about multithreading?

[@mak_whisk]

Thank you

[@Dennisvm82]

Thanks Brian! This was very interesting to watch :-D

[@pass1985]

The whole async for semaphores is in one block. Of course it will be executed in order, no semaphore would be needed there. Am I missing something?

[@comedycentral4333]

I have a question what will happen if we call the above code inside any completion handler in swift instead of viewDidLoad?

[@jensonjohn]

Thanks for the information 👍

[@wassmd84]

For shared resource example dispatchGroup.wait() can be used

[@leopapilaya]

Nice! Say, in a home page, if I want to display data section from top to bottom orderly, semaphore is the only way, right?

[@hasanirshad3535]

Hello Brian! Why did u set value 0? What if I set value 1?

[@bezeltipis]

Is this semaphores technic similar to async await concept?

[@mohanraj2640]

Hi I am a very big fan of your tutorial, In semaphore how to handle error cases, for example, consider there are 3 steps [API Call] if step 1 got error then step 2 and step 3 should not run. How to do this?

[@ciantim2996]

DispatchGroup doesn't guarantee that your tasks will complete with the same order as when you submit it. If you want ordered you need to use wait(timeout:) method

[@NikeshKannan]

Group.Enter = semaphore.signal which increments semaphore..

[@alexfrank5331]

10:31 that's a horrible concurrency code. At that point, you should just create a serial queue...

[@WestCoastAce27]

Good video - I now know how dispatch groups work - although a little too basic. Could do a little more than just print. Still not sure why they fired in that order. But will use that as a baseline for my application; hopefully it works. I'm trying to do the following: NSURLSESSION to make a req to a db server in the cloud; get the data (XML), parse, then use the data (x,y coordinates) to draw some bar graphs.

[@nixonlaine434]

Hey i just wanted to ask a question. I tryed using achor constraints and i did everything exactly like you did on screen but it wont let me use a anchor contraint from the view, which is the main view. i keep getting a error. it says ancesters dont match . EXC_BAD_ACCESS......have you ever had that problem???

[@adityavemuru25]

Hi Brian, I have a doubt. How many API calls can do at a given point of time ?

[@silviuisidor6184]

Amazing video!!

[@looly7555]

Why did you need to but it in a queue ?

[@LetsBuildThatApp]

Semaphores don't make a whole lot of sense when used on the main thread. Using it on a background queue allows for other things to run.

[@ArslanGhaffar]

hi whats difference btw dipatch barrier and semophores ?

[@ЕвгенийСычёв-п8у]

спасибо братишка за видосы)

[@ashwinichougale667]

Hey.. Please provide some tutorial on Apple Carplay Automaker

[@MaiTuongVy]

Hi Brian, I have a question about my case, I would like to find out how the differences between Semaphore and Dispatch Group resolve it The problem: I would like to download 100 images 1. Case 1: If I would like to wait until 100 images are completely downloaded, I should use DispatchGroup 2. Case 2: If I would like to download 5 images and show them in UI, then to the next 5 images. I should use Semaphore. Are they correct?

[@jasondubon]

why not use async await ?

[@MaiTuongVy]

What do you mean by using async await?@@jasondubon

[@MaiTuongVy]

Also, I would like to understand in case we want to choose between DispatchGroup and Semaphore. Are they the correct way to use them? (even maybe they are not good approaches)@@jasondubon

[@workflowinmind]

Bold outfit

[@davidlangley9287]

Very nice

[@vishalsharma7834]

Only use Semaphore when you need to manage order of execution of background task. Conclusion !!

[@gerardsk8ordie]

Is it correct to use a semaphore in the method "cellForItemAt" of a uicollectionview, in order to wait for the previous images to load before new ones can be loaded? I'm loading those images from a shared resource inside the code, as you said in the video.

[@gerardsk8ordie]

I'll respond to myself. yes.

[@hakankumdakc9233]

I guess its wrong if you do not change the shared resources. Although I don't know reading at the same time brokes program. Why don't you use operatingqueue?

[@Zainanjum100]

Awesome

[@rickisyourbro]

Man this is a good one! xiexie!

[@yeilmusic]

Better is dispatch queue barrier

[@everythingisamazing5765]

I was thinking the same... That seems to be the better solution

[@RostyslavKobizsky]

Thanks, but code style and API design is bad

[@stevenjemm]

Hey Brian, Using swift 5 Result type with DispatchGroups, would I need to include a dispatchGroup.leave() in the success case and the error case as well? Or just do it after the conditional statement checking success and errors?

[@sachinyadav-wb6nh]

Can u share an example ?

[@PS-dp8yg]

If you are a Javascript developer, this is equivalent to async/await.

[@rajanmaheshwari]

Hi Brian. In your case you are always receiving all 3 elements in shared resource in DispatchGroup irrespective of its order. Same thing I tried with 5 calls and not everytime my shared resource count to 5. It is sometimes 4 or 3 as well. No APIs are failing. Is it due to dispatchGroup member modifying the same state of array so fast that the other one also receive the old state while modifying. Here was the group fb query post I posted. facebook.com/groups/swiftiosdev/1761808553978003/. Can you answer or provide some direction or I have to use Semaphore (Last option). For me ordering is not important

[@RAKESHKUMAR-oe1bp]

Hello man , please one video uploads in firebase chat app create user chat send msg display on top user in Android function

[@mohanbabu3289]

please try to explain slowly then it will better for learners

[@sachinnautiyal4530]

On kzbin.info/www/bejne/bKOtf5J7eMlkjq8, if you change the value from 0 to >1, it will work perfectly fine as it is the number of threads you are allowing it to access. Right now it's 0 that's why it is not print anything. Why it started working was, when you executed 'semaphore.signal()' it incremented the count from 0 to 1, after that execution worked smoothly. Great video BTW, Waiting for more videos from you :)

[@GoingWalkabout]

Extremely useful, especially when I was struggling with 2 async fetches that need to complete before continuing.

[@ricardohochman]

Awesome video and nice explanation!

[@pawejanka3526]

This is gold 🎉